1. Field of the Invention
The present invention generally relates to sensors for collecting light from a target in the presence of ambient light and, more particularly, to improving the performance of such sensors, especially as used in electro-optical readers, such as laser scanners for reading indicia, such as bar code symbols, and in laser safety circuits for monitoring the output power of lasers, such as are used in laser scanners and in laser projection displays.
2. Description of the Related Art
Bar code readers are known in the prior art for reading various symbologies such as Universal Product Code (UPC) bar code symbols appearing on a label, or on the surfaces of an article. The bar code symbol itself is a coded pattern of graphic indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers electro-optically transform the graphic indicia into electrical signals, which are decoded into information, typically descriptive of the article or some characteristic thereof. Such information is conventionally represented in digital form and used as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like.
Readers of this general type have been disclosed, for example, in U.S. Pat. No. 5,600,121, and may employ a portable laser scanning device held by a user, which is configured to allow the user to aim the device and, more particularly, a scanning laser light beam, at a targeted symbol to be read. In moving laser beam readers known in the art, the laser light beam is focused by a lens or other optical components along a light path as a beam spot on a target that includes the bar code symbol. The moving-beam reader operates by repetitively scanning the beam spot in a scan pattern across the symbol by means of motion of a scanning component, such as a moving scan mirror placed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line, or a series of scan lines, or another pattern, across the symbol, or scan a field of view of the reader, or both.
Bar code readers also include a sensor or photodetector which detects light reflected or scattered from the symbol. The photodetector or sensor is positioned in the reader in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol. The light is detected and converted into an electrical signal. Electronic circuitry and software decode the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photodetector is converted by a digitizer into a pulse width modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded, based on the specific symbology used by the symbol, into a binary representation of the data encoded in the symbol, and subsequently to the information or alphanumeric characters so represented. Such signal processors are disclosed in U.S. Pat. No. 5,734,153.
Bar code readers are required to work under variable ambient lighting conditions, including indoor office lighting and outdoor sunlight, both indoor and outdoor lighting ranging from dim to bright. A combination of optical and electrical measures is employed to prevent bright ambient light from overwhelming the reader and preventing a successful decoding and reading of a symbol to be read. Such measures can be optimized for indoor or outdoor lighting at anticipated light intensity levels, but not for both. Thus, performance is sacrificed when a reader is required to work at non-optimized light intensity levels.
In a typical retro-reflective or non-retro-reflective reader, collection optics collect the laser light scattered from the symbol and concentrate the collected light onto a photodiode acting as the sensor. The collection optics also unavoidably collects ambient light and concentrates it on the photodiode. The photodiode generates an electrical composite signal proportional to the brightness of the total collected light from the symbol and from the ambient light. The composite signal is comprised of a data signal derived from the symbol and an ambient signal derived from the ambient light. If the ambient signal is high due to very bright ambient light, or if the ambient signal is on the same order of magnitude as the data signal as in the case where the reader is operating at the end of its useful working range, then the reader may not function because the data signal derived from the symbol will be lost.
Photodiodes are not only used in collection optics in a laser scanner, but are also used in laser safety monitoring circuits in laser scanners and laser projection display arrangements. As is known, a laser projection display arrangement projects a two-dimensional image in color on a screen based on a pair of scan mirrors which oscillate in mutually orthogonal directions to scan a plurality of differently colored laser beams, for example, red, blue and green, over a raster pattern of scan lines, each scan line having a number of pixels. A controller processes video data from a host, as well as control data with the host in order to form the image by selectively energizing and deenergizing a plurality of lasers that emit the laser beams.
In the event that a laser beam exits a device, such as a laser scanner or a laser projection display arrangement, the output power of each exiting beam is monitored by a photodiode and, if the monitored output power exceeds a safety value, then the safety circuit deenergizes the laser that emitted the beam. If the ambient signal is on the same order of magnitude as the monitored output power data signal, then the safety circuit may not function.
The prior art has proposed in FIGS. 9-11 of U.S. Pat. No. 5,923,021 preamplifier circuits for processing the output signal from a photodiode. These circuits include components that reduce the resistance of a feedback resistor, thereby reducing gain and worsening the signal-to-noise ratio, or that introduce shot or white noise from bipolar transistors and diodes which again worsen the signal-to-noise ratio. Hence, such preamplifier circuits are unsatisfactory for enabling a reader to perform reliably under all lighting conditions.
U.S. Pat. No. 7,128,264 discloses an arrangement for nulling ambient current in a photodiode circuit. However, this arrangement utilizes a low frequency, low pass filtering circuit that requires large bulky capacitors, and undesirably increases the size, weight and assembly cost of the arrangement.